It is generally known in the art that polyurethane elastomers have many applications, including, for example, producing industrial tires. A commonly used polyisocyanate for producing these polyurethane elastomers is 2,4-toluene diisocyanate (tolylene diisocyanate) or an isomeric mixture of 80 wt % of 2,4- tolylene diisocyanate and 20 wt % of 2,6-tolylene diisocyanate.
When a urethane wheel carries a load, work is applied to the urethane during compression or deflection. Upon removing the load most of the work is recovered as the tread returns to its original shape. A portion of the work is not recovered but is converted instead to heat. This is because there is no elastomer that is perfectly elastic.
In operation, urethane tires are subjected to repeated cycles of compression and relaxation which generate heat within the tire, causing an increase in tire temperature. The amount of heat generated is determined by the load on the tire, the tire rotation speed, and the amount of urethane in the tires.
Unless the heat generated can be dissipated rapidly, for example by conduction along the metal hub or by heat loss to the air, the temperature inside the elastomer will rise. Eventually the elastomeric tread of a rotating wheel may melt, delaminate from the hub or blow out due to the formation of gaseous decomposition products.
U.S. Pat. No. 4,089,822 discloses polyurethane elastomers made from polyurethane prepolymers. The prepolymers are made from 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate or a mixture of tolylene diisocyanates containing a major proportion of the 2,4-isomer. However, this patent does not recognize and therefore does not solve the problem of heat buildup in objects made from these polyurethane elastomers.
U.S. Pat. No. 3,963,681 discloses a polyurethane elastomer composition having improved static mechanical properties made from a polyfunctional isocyanate, curing agent and polyether. Among the polyfunctional isocyanates disclosed for use are tolylene-2,4-diisocyanate, tolylene-2,6-diisocyanate and their mixtures. The weight average molecular weight of the polyether is within the range of 1,000 to less than 4,500. Also, the molecular weight distribution curve of the polyether has at least two peaks. At least one peak is located in the higher molecular weight region and at least another one is located in the lower molecular weight region relative to the rheological critical molecular weight of the polyether. According to the patent, to obtain the improved properties of the compositions, one of the essential factors is the use of polyethers having the particular molecular weight range described (see, for example, the paragraph bridging columns 3 and 4, and paragraphs 1 to 3 of Column 4). This patent does not solve the problem of heat buildup in polyurethane elastomers. Furthermore, there is no recognition that by utilizing a particular level of 2,6-tolylene diisocyanate in the composition there is obtained a lower heat buildup tolylene diisocyanate based polyurethane elastomer.
A welcome contribution to the art would be a polyurethane elastomer that has low heat buildup characteristics (a dynamic mechanical property). Such a contribution is provided for by this invention.